@Override
 protected void onDraw(Canvas canvas) {
   for (int i = 0; i < balls.size(); ++i) {
     ShapeHolder shapeHolder = balls.get(i);
     canvas.save();
     canvas.translate(shapeHolder.getX(), shapeHolder.getY());
     shapeHolder.getShape().draw(canvas);
     canvas.restore();
   }
 }